This severely stupid. All mental conditions? Are you insane? You would be segregating and dividing the population more than what it already is, creating a huge disaster of inequality as "mental superiors" become the new master race. A major reason that there was a lot of African American inequality and brutality is because they didn't have their right to vote. Basically, not being able to vote restricts you from many jobs, judge, jury, lawyer, police officer, etc. Basically what was happening is that African Americans were unable to be represented or judged by other African Americans. Study Martin Luther King Jr. or Huckleberry Finn is also a very good example.
However, that's just some social barriers and inequality that could arise from banning people with mental conditions from voting. Let's talk about the absurdity and vagueness of a "mental disorder". A mental disorder can go from anorexia and claustrophobia to narcolepsy. Basically you would removing the right to vote from a citizen who doesn't like tight spaces, or someone who has OCD.
If you haven't got my point, it's pretty dumb